The development of portable electronic equipments in general, and mobile phones in particular, has revealed a significant interest for the use of Switched Mode Power Supply (SMSP).
Indeed, such SMPS system, in contrary to linear regulators, achieves a high efficiency—of the order of 90 or 95%—which is well beyond what is allowed by linear voltage regulator and therefore, electronic products manufacturer tend to generalize their use in a wide variety of portable devices, including mobile phones, MP3 players, Portable Device Assistant etc. . . .
Considering, for instance, the particular case of mobile phones, those device tend to incorporate many multimedia functions, including specific audio capabilities. A mobile phone is now likely to serve as a MP3 player and thus incorporates an audio amplifier which, for the purpose of saving the battery life, is preferably based on class AB or even class G using an voltage supply which is adjusted in accordance to the level of the audio signal to amplify.
The presence of such a class G audio amplifier, in addition to the more conventional communications circuits (RF transceivers, DSP etc. . . . ) have lead to the incorporation, within a same product, of two separate SMPS systems, a first one having a fixed output voltage for the purpose of supplying the main parts of the equipment and a second one, more specific to the audio amplification, providing different levels of output voltage for the power supply of the audio circuits.
Clearly, the use of two different SMPS systems causes a significant increases in the manufacturing costs of the product, at least because—as known by a skilled man—a SMPS device is based on the use of a external L-C network and that a L coil occupies a non negligible area.
As a conclusion, there is a desire for a new SMPS system improving efficiency and versatility for allowing integration, within a single integrated circuit, of various features and electronic circuits, such as a general purpose circuitry requiring a fixed voltage power supply and a specific audio circuit requiring at least two power supplies.
In addition, it is highly desirable that such SMPS system be simple and low cost to manufacture.